This invention relates generally to steering systems, and in particular, it relates to locking mechanisms for alternatively allowing and preventing the steering motion of wheels in an axle assembly. The invention has particular application in vehicles in which the rear wheels of the vehicle are steerable. In such vehicles, it is important, for safety purposes, to limit the steerability of the rear wheels when the vehicle reaches a certain level speed.
It is desirable for some vehicles, such as fire trucks and other large and special purpose vehicles, to have what is referred to as "all-wheel" steering. Examples of such systems are shown in significant detail in a number of patents, such as U.S. Pat. Nos. 5,417,299 and 5,111,901, both of which patents are assigned to the assignee of this invention and which are incorporated herein by reference.
Making the rear wheels of a steerable vehicle presents certain problems. For example, when the rear wheels are intended to be fixed, and not steerable, it is important that they be held rigidly in place, i.e. that they be play-free. In addition, when the wheels are placed in the locked and non-steerable condition, it is important that the wheels be aligned so that they are parallel with the longitudinal axis of the wheel. This is important so that when the front wheels are pointed straight ahead, the rear wheels will also be pointed straight ahead. Otherwise, the vehicle will proceed forward on an angle, sometimes referred to as "dog-tracking".
Another important consideration in the design of all-wheel steering systems is the prevention of inadvertent movement of the rear wheels, particularly at high speed. Even small amount of steering movement of the rear wheels of a vehicle, when the vehicle is proceeding at a high rate of speed, can cause the vehicle to overturn. Similarly, if there is any significant play in the steering motion of the rear wheels when the wheels are intended to be locked and non-steerable, can make it difficult to control the direction of the vehicle. This is due in part to the significant affect that a small amount of steering motion of the rear wheels can have on the direction of movement of the vehicle at high speeds.
Also, the presence of any significant amount of play can cause serious problems if the vehicle should happen to vibrate as a result of such play. This problem may arise at a particular speed at which the vehicle is driven, or when the vehicle is being towed with only the rear wheels in contact with the ground.
It is an object of the present invention to provide a locking mechanism for a steerable axle assembly in which play in the system is minimized or substantially eliminated.
It is another object of the present invention to provide a locking mechanism for a steerable axle assembly in which the mechanism has a fail-safe feature in which the lock is normally in the locked position.
Still another object of the present invention is to provide a locking mechanism for a steerable axle assembly which upon installation and at other times can be quickly and inexpensively adjusted so that the steerable wheels are pointing forward when the mechanism is in the locked position.
Yet another object of the present invention is to provide a locking mechanism for a steerable axle assembly in which there is little or no opportunity for the vehicle to vibrate when the mechanism is in the locked position, whether the vehicle is being operated under its own power or being towed.
Still another object of the invention is to provide a locking assembly which will, when actuated while the wheels are in a turned position, cause the lock to become engaged as soon as the wheels are returned to the straight ahead position.
The objects and advantages of the present invention are achieved with a steering lock assembly which includes a fluid-operated cylinder in which a moveable pin has been mounted. The pin has a tapered end which fits into a matching tapered hole when the mechanism is locked. A mechanical coil spring urges the pin into a locked position, and pressurized fluid, such as air, is used to move the pin out of the tapered hole to an unlocked position when steerability of the wheels of the assembly is desired. The coil spring ensures that if power in the vehicle is lost or interrupted, resulting in inability of the fluid system to act upon the pin, the spring will urge the pin in the direction of the tapered hole so that when the wheels are moved to the straight-ahead position, the mechanism automatically locks the wheels and prevents further steering motion of the wheels. In a preferred form of the invention, the tapered hole is formed in a sliding block supported by an anchor bracket, and the pin is held by a cylinder which moves in an arc as the wheels are turned. The pin is eccentrically mounted in the supporting cylinder, so that the combination of the rotation of the cylinder and movement of the sliding block provide ample adjustability in the relative positions of the main contact points of the locking mechanism.
In the preferred form of the invention shown herein, the pin and cylinder assembly is rigidly attached to the moveable wheel end of the steerable axle assembly, and the sliding block, in which is formed the tapered hole, is fixed to an anchor bracket mounted to a fixed or non-steerable portion of the steerable axle assembly.
The objects of the invention, as discussed above, and other aspects and advantages of the present invention, will be better understood upon a reading of the following specification, read in conjunction with the accompanying drawings.